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Query: UMLS:C0004134 (ataxia)
 15,886 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Cortical neurons in both superior (SPL) and inferior (IPL) parietal lobules are modulated by a variety of signals concerning planning and execution of eye and hand movement. Thanks to these properties, parietal neurons are ideally suited for eye-hand coordination during reaching. In SPL, a fundamental feature of neurons is the invariance of their directional tuning properties across tasks that require different forms of spatial relationships between the eye and the hand. In such conditions, the orientation of the preferred directions (PDs) of individual SPL cells cluster within a limited sector of space, the global tuning field (GTF), to be regarded as an ideal frame to dynamically match eye and hand signals on the basis of the orientation of their PDs. At the population level, the mean vectors of the GTF cover the direction continuum in a uniform fashion. These neurons are part of a parietal network richly interconnected with the premotor and motor areas of the frontal lobe. Thus, the reaching disorders of patients with optic ataxia might be interpreted as a consequence of the breakdown of the combinatorial mechanisms of the GTF of parietal neurons, and of their interplay with premotor cortex. In IPL, the main feature of eye and/or hand related neurons is the uneven distribution of their PDs, that mostly point toward the contralateral space. This anisotropy of the representation of directional motor space might explain the movement disorders that characterize directional hypokinesia in neglect patients. In conclusion, the study of the dynamic properties of parietal neurons and of their relationships with the premotor cortex via cortico-cortical connections provides a basis for an interpretation of movement disorders of parietal patients from a neurophysiological perspective.
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PMID:The cortical network for eye-hand coordination and its relevance to understanding motor disorders of parietal patients. 1645 34

The parietal mechanisms for online control of hand trajectory were studied by combining single-cell recording and reversible inactivation of superior parietal area 5 (PE/PEc; SPL) of monkeys while these made reaches and saccades to visual targets, when the target position changed unexpectedly. Neural activity was modulated by hand position, speed, and movement direction, and by pre- and/or postsaccadic signals. After bilateral muscimol injection, an increase in the hand reaction- and movement-time toward both the first and second targets was observed. This caused an increase in the time necessary for the trajectory correction, and therefore an elongation of the hand-path toward the first target location. Furthermore, hand trajectories were different in shape than control ones. An elongation of the eye reaction time to both first and second targets was also observed, which could partially explain the deficit of planning and correction of hand movement. These results identify the superior parietal lobule as a crucial node in the online control of hand and eye movement and highlight the role of the eye impairment in the emergence of the reaching disorder so far regarded as the hallmark of optic ataxia.
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PMID:Impairment of online control of hand and eye movements in a monkey model of optic ataxia. 2291 83

Optic ataxia (OA) is a neurological disorder that is characterised by misreaching to targets in the visual periphery. The anatomy of OA thus provides important information for the neural representation of visually guided reaching in humans. In 2005 a lesion mapping analysis of OA localised the critical lesion site at the parieto-occipital junction (POJ) (Karnath & Perenin, 2005). This work was accompanied by the discovery of a peripheral reaching module at the POJ in an fMRI study (Prado et al., 2005). The ostensible overlap between the territory typically affected in patients with OA and the findings of Prado et al. (2005) had a tremendous influence on the search for a cortical peripheral reaching module. However, a close inspection of the functional Magnetic Resonance Imaging (fMRI) study revealed that a comparison between reaching towards visible targets in the peripheral visual field and reaching to visible targets in the central visual field--which is the key aspect in clinical examinations of OA--was not conducted. Moreover, whereas main effects of reaching overlapped with the OA lesion site, specific interaction effects did not overlap. We performed a direct comparison between reaching to visible peripheral targets and reaching to visible central targets to address the inconsistencies between the aforementioned studies. Our analysis shows that Prado et al.'s study cannot be taken as evidence for a delineated module for peripheral reaching. In contrast to Prado et al. we found a combined system of POJ, IPS and SPL areas--the posterior human 7A, mIPS, V6A and the posterior IPS--with increased signals during reaching to peripheral targets.
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PMID:Revisiting the cortical system for peripheral reaching at the parieto-occipital junction. 2561 34